556 ANIMAL PHYSIOLOGY. 
instant the skin is being touched by one or two points is de- 
pendent on the part of the body with which the points are 
brought into contact. 
The following table will make this clear, the numbers indi- 
cating the distance at which the two points of a pair of com- 
passes must be apart in order that they shall not give rise to 
the judgment of one point of contact, but be recognized as two: 
Millimetres. 
Tip Of TON 000s esas wider a uedle sa iile were Pee aa 11 
Palm of last phalanx of finger..............-0005, 2°2 
Palm. of second phalanx of finger................ 44 
Dip OF NO86 Mividadn awa cuas wea sdawe aw sees 6'6 
Whitish part of lips............. cece eee eee eees 8'8 
Back of second phalanx of finger..............68. Ill 
Skin over malar bone ............ cece cee cece ees 15°4 
Back: Gf Wand aise cacc cra pawage yo eeeadeieswe ea 29°8 
ORCA 1 gas sah t onic iscsi ais pie wia stern ae Sia Sane 39°6 
GODT UTM asas2 tase coe cereal reds eteceenttel oie erent, Renae see 440 
Ba Ke cro.a2saass Aeaere 2a Ae teatent een on) NUS L wale oh Sita MEL LNTTY 66°0 
There seem to be areas of skin which give rise when pricked 
to the sensation of pain; but, whether we should distinguish be- 
tween tactile and pressure sensation by reference to correspond- 
ing spots, does not yet seem clear. 
Certain it is that exercise of these and all the senses greatly 
improves them, though it is likely that such advance must be 
referred rather to the central nerve-cells than to the peripheral 
mechanism. Careful comparison of blind and seeing children 
has shown that the blind, in forming their judgments, appar- 
ently from sensations derived through the skin, in reality use 
much collateral help, which is very variable and certainly 
widely different, according to the past experience and general . 
intelligence of the individual. 
We practically distinguish between a great many sensations 
that we can neither analyze nor describe, though the very 
variety of names suffices to show how much our interpretation 
of sense depends on past experience. 
We are always able to define the part of our bodies touched, 
and with great accuracy, no doubt, owing to the simultaneous 
use in early months and years of our lives of vision and the 
senses resident in the skin. 
There are, however, transient illusions of sense which illus- 
trate the remark just made. If a small marble be vlaced be- 
